1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave filter and a method of manufacturing the same, especially to a surface acoustic wave device electrode.
2. Description of the Related Art
In recent years, surface acoustic wave devices having the advantages of low insertion loss and small size have been widely used as a resonator or a filter for mobile communication systems. In general, the surface acoustic wave device includes a single crystal piezoelectric substrate (piezoelectric single crystal substrate) and a comb electrode (inter digital transducer, hereinafter referred to as an IDT) formed on the piezoelectric substrate. As a material for the piezoelectric single crystal, quartz, lithium tantalate (LiTaO3), or lithium niobate (LiNbO3), etc. is used, and especially for an RF band filter, a 64 degrees rotated Y-cut LiNbO3 having a large electromechanical coupling coefficient or a 32 degrees to 44 degrees rotated Y-cut LiTaO3 having a large electromechanical coupling coefficient and a rather small frequency temperature coefficient has often been used.
Since excellent micro-machinability, small density to make the loading mass effect by electrode small, and low resistivity to make the insertion loss small, are required for material of a comb electrode in the surface acoustic wave device, aluminum or an aluminum alloy is generally used.
When operating a surface acoustic wave device, repeated stress proportional to frequencies is applied to an IDT electrode. It is known that hillocks or voids are created in the IDT by repeated application of stress causing stress-migration, and that the hillocks or voids make the filter characteristics deteriorate. In general, the more the electric power applied, and the higher the operating frequency, the resistance to stress-migration, or power durability, of the IDT electrode becomes low. Thus, an electrode material excellent in power durability is required for a surface acoustic wave filter, especially for a duplexer for cellular phones to which large power is applied at the RF band from 800 MHz to 2 GHz.
Hitherto, developing an electrode material for a surface acoustic wave device having a migration durability, the following methods have been proposed (1) addition of impurities, (2) micro-grain size, (3) multi-layering, (4) segregation, (5) development of high texture, (6) single crystal, and so on. The respective development methods will be explained hereinafter.
The first “addition of impurities” is a method of enhancing the rigidity of the electrode to improve power durability by adding a small amount of copper (Cu), silicon (Si), titanium (Ti), palladium (Pd), or the like to aluminum (Al). Though the power durability of the electrode is improved as the concentration of atoms added increases in general, there arise problems such as increase of insertion loss due to increase of resistivity, etching residue at the time of electrode processing, or the like. Accordingly, it is undesirable to add atoms in high concentration. Therefore, it is difficult to achieve sufficient power durability required for the surface acoustic wave filter for a duplexer by only adding impurities to aluminum.
The second “micro-grain size” is a technology disclosed, for instance, in Patent Document 1, and is a method of enhancing the rigidity of the electrode (similarly to addition of impurities) to improve power durability by making the average grain size of the electrode film small (in Patent Document 1, the average grain size is reduced to a value in the range from fiftieth to fifth of the size of the electrode finger). It is described that in order to make the grain size small, it is sufficient to add at least one metal selected from titanium (Ti), palladium (Pd), copper (Cu), niobium (Nb), nickel (Ni), magnesium (Mg), germanium (Ge), silicon (Si), cobalt (Co), zinc (Zn), lithium (Li), tantalum (Ta), gold (Au), silver (Ag), platinum (Pt), chromium (Cr), hafnium (Hf), zirconium (Zr), cadmium (Cd), tungsten (W) and vanadium (V) at wt % of 20 or less. However, reduction of grain size by the addition of impurities cannot avoid the increase in electrode resistance, which causes the undesirable increase of the insertion loss.
The third “multi-layering” is a method of enhancing the rigidity of an electrode film to improve power durability by laminating aluminum or aluminum alloy on a metal layer different from aluminum (Al), or by alternately laminating an aluminum layer and a metal layer other than aluminum (Al). As a typical example of the former, Patent Document 2 proposes an electrode structure which is “an aluminum alloy film laminated on a titanium buffer layer of 100 to 200 nm in thickness, in which the titanium film thickness accounts for 25 to 60% of the electrode thickness” It is described that since the texture of the aluminum alloy is lowered on the thick titanium buffer layer and the grain size becomes smaller, the stress on the Al or Al alloy film decreases, which results in improvement of the power durability. However, since the titanium buffer layer having a large resistivity occupies the large portion of the electrode, the resistance of the electrode becomes large, which is undesirable in terms of insertion loss.
Whereas, as a typical example of the latter, Patent Document 3 discloses an electrode having a laminated structure, in which respective two or more layers of aluminum layer and conductive layer X having a large elastic constant than aluminum (Al) are laminated alternately, adjusting the thickness of the aluminum layer and the X layer according to the stress load along the direction of the film thickness. However, as described in an embodiment of Patent Document 3, since the resistance of the electrode increases rapidly by performing multi-layering, it also brings about a problem in terms of insertion loss. In addition, since the etching properties of the aluminum layer and the X layer differ from each other, there is a problem of difficulty in size and shape control of the electrode cross section in the patterning process of comb electrode.
The fourth “segregation” is a method of enhancing the rigidity of an electrode film to improve power durability by segregation of metals other than aluminum (Al) in the aluminum alloy film. Patent Document 4 discloses “an aluminum) electrode formed on a TiN buffer layer or a titanium buffer layer, in which at least one kind of metal among copper (Cu), tantalum (Ta), tungsten (W) and titanium (Ti) is segregated in the film with a grain size of 100 to 1000 nm”. When the electrode film is etched into a comb electrode shape, the segregated grains cannot be etched, which brings about a problem of short circuit between comb electrodes.
Patent Document 5 discloses “a multi-layered electrode including a buffer electrode film, an aluminum alloy electrode film, and an aluminum alloy electrode film containing easy-to-diffusion element on a piezoelectric substrate”. The uppermost aluminum alloy electrode containing easy-to-diffusion element is made of a material including at least one selected from a group composed of copper (Cu), silver (Ag), gold (Au), nickel (Ni) and magnesium (Mg), which are easily diffused into grain boundaries in the aluminum alloy film as a main component, and the selected metal is diffused and deposited into the grain boundary of the aluminum alloy layer from the uppermost layer. Since this electrode has a structure such that the grains of the aluminum alloy, which is served as a principal electric conducting path, are surrounded by the grain boundary of the easy-diffuse metals having a high resistivity, the electric resistance of the electrode becomes high, which is undesirable in terms of insertion loss.
The fifth item of “development of high texture” refers to an electrode in which the (111) texture of aluminum or aluminum alloy is enhanced, as disclosed, for instance, in Patent Document 6. In a thin layer having a high (111) texture, though the <111> axes of the respective crystal grains are arranged almost uniformly in the normal direction of the substrate, their in-plane directions are dispersed randomly. Such a film in which only particular crystal axes thereof are arranged uniformly in the normal direction of the substrate, is called a uniaxial texture film. Though the diffusion routes of aluminum atoms or vacancies moving by stress-migration are mainly along grain boundaries, since the in-plane is covered by grains having (111) planes in the (111) texture film, the in-plane shows a honeycombed grain boundary structure, and the grain boundary takes a net structure formed by almost triple points. In such a case, the flux of the aluminum atoms or vacancies diffusing along the grain boundaries is easily balanced. This is considered to be the reason why the power durability is remarkably improved, compared with a poly-crystal film in which principle directions are completely random, or a low texture film.
Patent Document 6 discloses that use of a metal buffer film having 1 to 50 nm in thickness, selected from vanadium (V), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), yttrium (Y) and chromium (Cr) can form an aluminum or aluminum alloy film strongly textured in the (111) direction. Patent Document 7 discloses that an aluminum or aluminum alloy film also strongly textured in the (111) direction is formed on a buffer film selected from boron (B), carbon (C), silicon (Si), germanium (Ge), SiC, BN and SiN. Furthermore, Patent Document 8 discloses that an aluminum or aluminum alloy film strongly textured in the (111) direction can be formed on a metal buffer film having the thickness of 50 nm or less, selected from tantalum (Ta), niobium (Nb), titanium (Ti), tungsten (W), molybdenum (Mo), nickel (Ni), hafnium (Hf) and scandium (Sc). It is described to be important that the buffer film is in an amorphous state.
Patent Document 9 discloses that a (111) high texture aluminum film can be formed by using a multi-layered structure composed of aluminum (Al) and a metal other than aluminum (Al) as a buffer film. Since the electrode resistance of a uniaxial high textured electrode film is less likely to get high compared with other methods described above (unless a metal possible to easily diffuse into aluminum (Al) is used for a buffer film), the possibility to increase the insertion loss is small. However, when considering recent circumstances requiring downsizing of a surface acoustic wave filter for a duplexer, development of an electrode material showing a higher power durability than that of the uniaxial high texture electrode is needed.
The sixth “single crystal” is a method to grow a film such that a specific crystal direction in a specific lattice plane of the piezoelectric substrate matches with a specific crystal direction of a specific lattice plane of the aluminum electrode (this is called an epitaxial growth). Since the crystal directions of the respective grains of the aluminum or aluminum alloy are arranged in a fixed direction almost uniformly, not only in those planes along the normal direction of the substrate, but also in the in-plane, it takes a state of no lattice boundaries, or a state in which grain boundary diffusion is restrained. Therefore, aluminum atoms or vacancies are to diffuse in a so-called lattice diffusion mode. The diffusion speed is extremely low in the lattice diffusion mode compared with that in the boundary diffusion mode. It is said that this is the reason why the power durability of a single crystal film can be more improved than that of the uniaxial high texture film.
Various methods to grow a single crystal aluminum (or aluminum alloy) film have been disclosed for the case of using quartz as a piezoelectric substrate. For instance, Patent Documents 10 and 11 disclose an aluminum (311) epitaxial film on a 25 degree rotated Y-cut quartz, and Patent Documents 12 to 14 disclose an aluminum (111) epitaxial film on a 4 to 30 degree rotated Z-cut quartz.
Materials on which an epitaxial aluminum film is obtained by the conventional methods described in Patent Documents 10 to 14 are practically limited to only a quartz substrate. A piezoelectric substrate used for an RF band surface acoustic wave filter which requires excellent power durability is LiNbO3 or LiTaO3. The followings are well known methods to grow a single crystal or an epitaxial aluminum film on those piezoelectric substrates.
Patent Documents 15 and 16 are conventional methods to directly grow single crystal aluminum (Al) on piezoelectric substrates made of LiNbO3 or LiTaO3. The epitaxially grown aluminum (111) film on a 36 degree rotated Y-cut LiTaO3 is disclosed in the former, and the aluminum (111) film grown epitaxially on 32 to 68 degree rotated Y-cut LiTaO3 and LiNbO3 substrates is disclosed in the latter. There is a description that an epitaxial aluminum film can grow on LiTaO3, LibO3, or Li2B4O7 substrates, as well as on a quartz substrate, but nothing is mentioned on what plane and in what plane direction the aluminum film epitaxially grows.
Patent Document 17 discloses a method of manufacturing an epitaxial aluminum film “by growing the film while irradiating ion beams having an ion current density of 0.01 to 10 mA/cm2 with ion energy of 200 to 1000 V”. However, the difficulty of directly forming single crystal aluminum (Al) on a substrate, and the extremely low production yield are problems existing in this method. This is caused by a condition that the original lattice plane of the piezoelectric substrate is covered by a processed surface layer having a disordered crystal structure created by a polishing process on the substrate surface. In order to remove the influence of the surface layer having the disordered crystal structure of the substrate surface, Patent Document 18 discloses a method to grow a single crystal aluminum (Al), “by uniformly fabricating a microscopic hemispherical island structure on the substrate surface”, by means of wet etching or dry etching using fluorine related gas, though no description is made on its direction of growth. Patent Document 19 discloses a method of “growing a single crystal aluminum film after removing the substrate surface processed layer by means of ion beam etching by arranging an ion source so that the direction of the irradiating ion beams is within ±20° from a position parallel or perpendicular to the axial direction perpendicular to a lattice plane having a low index plane closest to the surface of a LiTaO3 substrate, or a LiNbO3 substrate”.
This kind of substrate preprocessing has problems of not only an increase in the number of manufacturing processes, but also the difficulty of stable growth of a single crystal aluminum film owing to fluctuations of the preprocessing or the like. Therefore, it is not likely to be easy to directly grow the single crystal aluminum film on a LiTaO3 substrate or a LiNbO3 substrate. Accordingly, a method to grow a single crystal aluminum film via some buffer layer, not to directly grow the single crystal aluminum film on the LiTaO3 or LiNbO3 substrates, has been proposed. Insertion of a buffer layer gives the merit of enabling the growth of a single crystal aluminum film on a substrate with increased stability, compared with the case of directly growing the single crystal aluminum film on the substrate, because it relaxes lattice mismatching between the substrate crystal plane and the epitaxially growing aluminum plane. Therefore, it can be said to be preferable to use a buffer layer in terms of production yield.
Patent Document 20 discloses that it is possible to grow a (110) single crystal aluminum film by growing aluminum (Al) on a 64 degree rotated Y-cut LiNbO3 substrate via a titanium buffer layer. It is described that there is an epitaxial relation such that the normal directions of a titanium (001) plane and an aluminum (110) plane are coincided with the direction perpendicular to the substrate, and both of the titanium (Ti) and aluminum (Al) are single crystal films displaying only spots in a selected-area electron diffraction. Patent Document 21 describes that when a titanium buffer layer is used on a 38 to 44 degree rotated Y cut LiTO3 substrate, a (112) single crystal aluminum film grows in an epitaxial relation so that the normal direction of the aluminum (112) plane coincides with the perpendicular direction of the substrate surface.
Patent Document 22 discloses that when TiN is used as a buffer layer on a 33±9 degree rotated Y-cut LiTaO3 and LiNbO3 substrates, a (311) single crystal aluminum film can be grown in a tilting state by 9±9 degree to the substrate surface, and when a structure composed of two layers of TiN/Ti (on a substrate) is taken as a buffer layer, a (111) single crystal aluminum film can be grown in a tilting state by 9±9 degrees to the substrate surface.
These single crystals or epitaxial aluminum films are single crystals in which an aluminum specific crystal plane grows in parallel or in a tilting state by 9±9 degrees to a LiTaO3 substrate surface or a LiNbO3 substrate surface as described in Patent Document 20 (110), in Patent Document 21 (112) and in Patent Document 22 (311 or 111).
Whereas non-Patent Document 1 discloses that aluminum (111) performs twin growth on a 63 to 70 degree Y-cut LiNbO3 substrate via a titanium buffer layer. It is reported from an analysis using a pole figure that an aluminum (111) plane performs twin growth in parallel to a (001) plane (also referred to as a Z plane) of LiNbO3, irrespective of Y-cut angles. That the aluminum (111) plane is in parallel with the LiNbO3 (001) plane, means that the direction of aluminum <111> tilts to a lithium niobate substrate surface by (90—Y-cut angles)°. The twin growth is a stacking fault, and indicates a state in which two crystals are in a mirror symmetry as for a specific lattice plane. In other words, a (111) twin crystal aluminum film is not a complete (uni-) single crystal layer, but is a pseudo-single crystal layer which consists of a mixture of two kinds, that is, one aluminum (111) single crystal grain (domain), and the other aluminum (111) single crystal grain (domain) which is formed by rotating the former by 180° in the in-plane. The (111) twin crystal growth is characterized by that a pole figure depicted by plotting (100) poles takes a six-time symmetrical pattern as shown in FIG. 1 or FIG. 3 in non-Patent Document 1.
Patent Document 23 discloses “an electrode having a twin structure in which a diffraction pattern observed in a pole figure has a plurality of (111) centers of symmetry”. It is described that in order to realize such a twin structure having a plurality of centers of symmetry, provision of a process to perform wet-etching on 36 to 42 degree rotated Y-cut LiTaO3 or LiNbO3 substrates, and provision of a titanium or chromium buffer layer are essential. The inventors report in Patent Document 23 the analysis of growth direction of an aluminum film having the thickness of 150 nm by measuring a pole figure in non-Patent Document 2. The aluminum film is deposited at the lowered substrate temperature to room temperature, after depositing a titanium buffer to the thickness of 10 nm at the substrate temperature of 180° C. on a 38.5 degree rotated Y-cut LiTaO3 substrate. According to this report, it is described that aluminum (111) grows in a twin crystal state in parallel to a (001) plane of LiTaO3, similarly to the twin growth of aluminum (111) on the LiNbO3 substrate in the above-described non-Patent Document 1. Therefore, it is natural to consider that “the aluminum (111) twin crystal having a plurality of centers of symmetry” also grows in parallel to the (111) plane of LiTaO3.
That is, the domain (111) a of the aluminum film obtained in Patent Document 23 grows in the direction of (90−38.5)° to the normal Z1 of the piezoelectric substrate b, or in a direction deviated by 51.5° from the normal Z1 as shown in FIG. 14, and it can be said that two domains (111) being in mirror symmetry are mixed in the domain (111) group vertically growing in the figure.
In order to form an aluminum electrode having the (111) twin structure having a plurality of centers of symmetry, as described in Patent Document 23, before forming a buffer electrode layer, etching to expose a lattice plane on the piezoelectric substrate surface is needed with at least one kind of etchant selected from the group composed of phosphoric acid, pyrophosphoric acid, benzoic acid, octanoic acid, hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, buffer hydrofluoric acid, and potassium acid sulfate. Preprocessing by wet etching causes a problem of poor yield due to a difficulty to control the substrate surface conditions with good reproducibility. It is undesirable in terms of resultant increases in the manufacture process.
Patent Document 1: Japanese Patent Application Laid-open No. Hei 6-6173
Patent Document 2: Japanese Patent Application Laid-open No. 2002-368568
Patent Document 3: Japanese Patent Application Laid-open No. Hei 9-135143
Patent Document 4: Japanese Patent Application Laid-open No. 2005-39676
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Patent Document 8: Japanese Patent Application Laid-open No. 2001-94382
Patent Document 9: Japanese Patent Application Laid-open No. 2003-188672
Patent Document 10: Japanese Patent Application Laid-open No. Hei 3-14308
Patent Document 11: Japanese Patent Application Laid-open No. Hei 3-48511
Patent Document 12: Japanese Patent Application Laid-open No. Hei 6-132777
Patent Document 13: Japanese Patent Application Laid-open No. Hei 7-170145
Patent Document 14: Japanese Patent Application Laid-open No. Hei 8-28272
Patent Document 15: Japanese Patent Application Laid-open No. Hei 5-183373
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Patent Document 20: Domestic Re-Publication of PCT International Publication for Patent Application WO99/16168
Patent Document 21: Domestic Re-Publication of PCT International Publication for Patent Application WO00/74235
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Electrode materials having low resistivity and power durability are needed for a surface acoustic wave filter used for a duplexer for 800 MHz to 2 GHz RF band. For this usage, a single crystal aluminum film which is excellent in stress-migration resistance is considered to be most promising. However, since the surface of a 64 degree rotated Y-cut LiNbO3 substrate or a 32 degree to 44 degree rotated Y-cut LiTaO3 substrate is not coincident with a low index lattice plane indispensable for epitaxial growth, it is not easy to realize the epitaxial growth of a single crystal aluminum film on such a piezoelectric substrate. As described in [Background of the Invention], it is understood that it is difficult to grow the single crystal aluminum film on a LiTaO3 or LiNbO3 substrate with good reproducibility even if preprocessing of a substrate or introduction of a buffer layer are conducted, because single crystal aluminum films having various epitaxial growth directions are obtained even on the same substrate.